III-nitride crystals find applications in light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs), in electronic devices such as rectifiers, bipolar transistors, field-effect transistors, and high-electron-mobility transistors (HEMTs), in semiconductor sensors such as temperature sensors, pressure sensors, radiation sensors, and visible-blind ultraviolet detectors, in surface-acoustic-wave devices (SAW devices), in acceleration sensors, in micro-electromechanical system parts (MEMS parts), in piezoelectric vibrators, in resonators, and in piezoelectric actuators. III-nitride crystal of low dislocation density is being sought after in such applications, in order to improve the performance characteristics of the various devices.
Proposed methods of manufacturing III-nitride crystal to have low dislocation density include the following techniques. Japanese Unexamined Pat. App. Pub. No. 2003-183100 (“Patent Document 1” hereinafter) discloses a facet-growth method in which a mask patterned in regular stripes is provided on an undersubstrate, and atop the mask GaN crystal is vapor-deposited while forming plurally faceted, linear V-grooves and sustaining the grooves, and whereby dislocations within the GaN crystal are gathered directly beneath the V-grooves (the regions into which dislocations concentrate being termed “crystal-defect gathering regions”), reducing the surrounding dislocation density.
With the just-described facet growth method of Patent Document 1, dislocation density in the region apart from the crystal-defect gathering regions can be reduced to a level of 1×105 cm−2, yet the dislocation density in the crystal-defect gathering regions will be high. Moreover, in such defect-gathering regions, the polarity in the <0001> directions is often inverted with respect to the crystal region apart from the regions where defects gather. The consequent challenges of epitaxially growing a III-nitride semiconductor layer onto a GaN crystal substrate obtained by facet growth mean low semiconductor device yields.
Meanwhile, Japanese Unexamined Pat. App. Pub. No. H10-312971 (“Patent Document 2” hereinafter) discloses an epitaxial lateral overgrowth (ELO) technique in which an undersubstrate being a GaN thin film formed onto, e.g., sapphire is prepared, from atop the undersubstrate a mask of, e.g., SiO2, having apertures is formed, and GaN crystal is epitaxially grown laterally onto the mask through the apertures.
The just-described ELO technique of Patent Document 2 makes lateral crystal growth without occurrences of strain and cracking possible, thus reducing dislocation density by comparison with implementations in which GaN crystal is grown directly onto an undersubstrate; yet fresh dislocations arise where the laterally growing crystal coalesces, which is prohibitive of getting the dislocation density down to under 1×107 cm−2. On this account, making such GaN substrates practicable as substrates for LDs has been problematic.
In another proposal, the detailed description in U.S. Pat. No. 5,868,837 (“Patent Document 3” hereinafter) discloses a sodium flux technique in which, at a temperature of some 600° C. to 800° C. and under a nitrogen atmosphere at a pressure of some 5 MPa, nitrogen gas is supplied to a Ga—Na melt to grow GaN crystal.
The just-described sodium flux method of Patent Document 3 makes it possible to grow low-dislocation-density, low-defect GaN crystal under temperature and pressure conditions relatively moderate for a liquid-phase technique, yet the crystal growth rate is slow, which is prohibitive of obtaining bulk GaN crystal.
In still another proposal, Japanese Unexamined Pat. App. Pub. No. 2004-221480 (“Patent Document 4” hereinafter) discloses forming a starting substrate, in which differing polarity A, B domains coexist, into a skeletal substrate in which the entirety or a portion of either one of the polarity domains has been removed by etching, and by growing onto the skeletal substrate crystal of the same material as the substrate, filling in the given removed portion with crystal having the other polarity, to obtain crystal whose entire surface has the other polarity. With the method of Patent Document 4, however, because the domains of the one polarity are domains in which the polarity in the <0001> directions is inverted with respect to the domains of the other polarity, when those domains are filled in by crystal growth using a vapor-phase technique, growth in which the polarity of those domains (the one polarity) is inherited occurs. Thus, to the extent that the entire crystal surface is to be covered with crystal having the other polarity, the entirety or a portion of the domains of the one polarity in the substrate must be deeply removed by etching, which complicates the manufacturing method.    Patent Document 1: Japanese Unexamined Pat. App. Pub. No. 2003-183100.    Patent Document 2: Japanese Unexamined Pat. App. Pub. No. H10-312971.    Patent Document 3: Detailed Description in U.S. Pat. No. 5,868,837.    Patent Document 4: Japanese Unexamined Pat. App. Pub. No. 2004-221480.